Interesting Events

Here we describe recent, rapid changes in the cryosphere that are newsworthy. They may or may not be significant in a climate context.

Larsen C Ice Shelf Sheds a Big One!

William Straka III and Jeff Key, 15 July 2017; updated 3 August, 8 September

Antarctica just shed a very large piece of the Larsen C ice shelf. A block of ice the size of the U.S. state of Delaware broke off sometime between July 10 and 12. This block of ice is now an iceberg named A-68. Scientists have been watching the fracture grow in length and width since 2016. The 200m-thick iceberg is estimated to weigh about 900 billion tonnes (one trillion tons) and span 5800 sq km (2239 square miles). The Larsen C ice shelf was the fourth largest in Antarctica. It is now 12% smaller and may now be the fifth or sixth largest. This calving event has created one of the largest icebergs on record, though its size is not unprecedented in recent decades. The largest iceberg observed by satellites, called B-15, broke from the Ross Ice Shelf in 2000. It was approximately 11,000 sq km in area. In 1956 a US Navy icebreaker came across an object roughly 32,000 sq km. The location of the Larsen C ice shelf is shown in Figure 1.

There has been considerable media coverage of the event. See some of the resources listed below. Here we focus on satellite imagery from the Visible Infrared Imaging Radiometer Suite (VIIRS) on the NASA/NOAA Suomi National Polar Orbiting Partnership satellite (S-NPP).

Figure 1: Approximate location of the Larsen C ice shelf in Antarctica (left) and the Antarctic Peninsula (right). (Based on maps from Google Earth.)

July 12: It is winter in Antarctica now so there is very little or no sunlight. VIIRS has a unique "day-night band" (DNB) that provides visible imagery in the presence of moonlight. The Waning Gibbous moon (~90% illumination) provided enough light to clearly see the iceberg that had broken off the Larsen-C ice shelf. Infrared imagery is also available. Figure 2 provides both DNB and infrared images.

Figure 2: The iceberg that broke off the Larsen C ice shelf on 12 July 2017 at 03:26 UTC, as viewed by the VIIRS day-night band (DNB) on the left, and a false-color infrared band (11 microns) on the right. Note that in these and the following images south is toward the top. (Courtesy of the Cooperative Institute for Meteorological Satellite Studies (CIMSS), University of Wisconsin-Madison.)

July 14: S-NPP imagery was acquired at 04:49 UTC (Figure 3). The Waning Gibbous moon (~83% illumination) continued to provided enough light to clearly see the iceberg that had broken off from the Larsen-C ice shelf. Fortunately, the area was clear again, showing that there appears to be a slight expansion of the lead on the north side of the iceberg, as indicated by the dark, open water, in the DNB image as well as the warmer pixels in the 11um channel. Of particular interest is the additional fracture of a relatively small iceberg on the north end of A-68 (bottom portion in Figure 3).

Figure 3: As in Figure 1 but for 14 July 2017, zoomed in to show the smaller iceberg breaking off of A-68.

Figure 4 provides animations of the VIIRS day-night and infrared bands at two spatial scales over the July 12-14 period.

Figure 4: Animations over the period 12-14 July 2017 from the VIIRS day-night band (DNB) in the top row and the VIIRS 11 micron (thermal) band in the bottom row. The figures on the the right are zoomed in on the iceberg. Note that south is toward the top.

July 16: A-68 breaks off another piece, as shown in Figure 5.

Figure 5: False-color VIIRS infrared band for 16 July 2017 at 03:52 UTC showing another fracture of A-68.

August 3: There has not been a significant amount of movement over the last several weeks, though there has been additional fracturing of A-68 (Figure 6).

Figure 6: The position and condition of A-68 on 3 August 2017 at 03:14 UTC, as viewed by the VIIRS day-night band (DNB) on the left, and a false-color infrared band (11 microns) on the right.

Is this a sign of climate change? Not necessarily. Calving of ice shelves is a natural process. Ice sheets and glaciers are fluid, always moving forward if there is an accumulation of snow in their upper reaches. This motion can be accelerated by a changing climate, but it is difficult to determine the degree to which this particular event was a result of a warming climate. Furthermore, calving of an ice shelf does not significantly affect sea level because the ice shelf was already floating (which is what makes it a "shelf"). However, if the Larsen C ice shelf were to collapse, it would allow blocked glaciers to flow into the ocean, potentially and eventually raising sea level by 1 cm.

September 8: Comparing Figures 7, 6, and 3 illustrates that there has been little movement over the last month apart from some rotation (counter-clockwise in Figure 7). There has been a recent increase in open water (sea ice movement or retreat) around A-68.

Figure 7: The position and condition of A-68 on 8 September 2017 at 03:41 UTC, as viewed by the VIIRS day-night band (DNB) on the left, and a false-color infrared band (11 microns) on the right.

More information is available from the mainstream media and elsewhere: